Method of sintering compacts



Jan. 17, 1961 E. D. NORTH ETAL METHOD OF SINTERING COMPACTS Filed Sept.18, 1958 FIGI.

FIGZ.

United States Patent METHOD or SINTERING COMPACTS Edward D. North,Waterloo, Ill., and James A. Rode, Crestwood, and Gervaise W. Tompkin,Crystal Lake, Mo., assrgnors to Mallinckrodt Chemical Works, St. Louis,Mo., a corporation of Missouri Filed Sept. 18, 1958, Ser. No. 761,798

4 Claims. (Cl. 75-223) The present invention relates to powdermetallurgy and more particularly to the production of sintered fuels foruse in nuclear reactors.

Briefly, the method of the present invention relates to the formation ofsintered compacts of metallic oxides and metals by continuous means. Themethod employs unsintered compacts of at least one powdered metalsubstance and sinters these without sacrifice of density of product andwithout substantial fracture of the compacts or agglomeration thereof.The method is particularly useful for sintering uranium dioxide compactswhich are to be used in fuel elements for nuclear reactors.

Among the several objects of this invention may be noted the provisionof improved methods for sintering compacts of powdered metals ormetallic oxides; the provision of methods of the character describedwhich provide for simultaneously removing heat-fugitive binders,lubricants, orother temporary additives; the provision of methods of thecharacter described which can be carried out with, simple equipment andmaterials; the provision of methods of the character described which areeconomical with respect to power consumption; and the provision ofmethods of the character described which are adaptable either tocontinuous or semi-continuous operation. Other objects and features willbe in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the methods hereinafter described,the scope of the invention being indicated in the following claims.

In the accompanying drawing, in which one form of apparatus useful incarrying out the invention is shown,

Fig. 1 is an elevation partly in section; and,

Fig. 2 is a perspective of a compact.

Corresponding reference characters indicate corresponding partsthroughout the drawing.

It is known to form cold compacts of powdered uranium dioxide, usuallycylindrical in form, by compressing the powder in suitable dies, andthen sintering the compacts at elevated temperatures to obtain denseforms of uranium which are useful in fuel elements for nuclear reactors.

It has also been customary to prepare the uranium dioxide compactsdescribed above with the aid of a small amount, in the order of 0.1 toabout 6%, of a heat-fugitive binder such as, for example, paraffin. Theuranium dioxide may be enriched or depleted with respect to U or it maycontain the naturally occurring proportions of isotopes. While theinvention will be more fully described in terms of preparing suchproducts, its usefulness is not so limited. The purpose of the binder istwofold. First, it increases the density of the compacts by reducinginterparticle friction and friction between the particles and the wallsof the die in which the compacts are formed. Second, it increases thegreen, i.e., unsintered, strength of the compacts, thus allowing them tobe handled with less chance of their becoming broken or deformed beforesintering. The binder is then driven off during the subsequent sinteringoperation. In the case of uranium 2,968,551 Patented Jan. 17, 1961dioxide, it is important for the binder to be one which is completelyremoved from the compacts without leaving any carbonaceous residue whichmight form uranium carbide during the sintering operation. The compactsare next heated to a temperature suflicient to drive off the binder,which in the case of paraffin is in the neighborhood of 400-500" C.Thereafter, the temperature is increased to 1500 C. or above to completethe sintering of the compacts.

The operations of dewaxing and sintering the compacts are often carriedout in separate furnaces, the compacts being transferred from one to theother to avoid the time lost in cooling and reheating the furnace whenonly one furnace is used. To guard against contamination and also tofacilitate handling the green compacts, they are usually supported inmolybdenum boats during the de waxing and firing operations.

The conventional methods for making such sintered compacts are bothlaborious and expensive. The necessity for twice loading and unloadingfurnaces for each batch of compacts is obviously time-consuming, and theuse of molybdenum containers is expensive and their capacity is limited.Also, conventional continuous furnaces are usually not practical becauseof their high cost.

From the foregoing discussion it is apparent that there is need for asimpler and more efficient method of sintering uranium dioxide compacts.Such a method should be readily adaptable to either large or small scalemanufacture, and it should be capable of either continuous orsemi-continuous operation using simple and readily available equipmentand materials.

According to the present invention, the green, uranium dioxide compactsare dispersed in a body of roughly spherical, chemically pure,free-flowing granules of a more refractory material such as fusedalumina or zirconia, the granules being of such a size that theyeffectively surround each compact and prevent it from coming in contactwith other compacts or with the walls of the furnace. This mixture isthen allowed to gravitate slowly through a vertical, substantiallytubular furnace filled with the mixture, preferably in a protectiveatmosphere such as that provided, in the case of uranium dioxide, by acountercurrent stream of hydrogen or steam. The

upper portion of the furnace is provided with heating means so that thecompacts are gradually heated to their sintering temperature as theymove through the furnace. This temperature is insufficient to sinter therefractory granules. During this period the binder, if one is present,is driven off and carried away with the stream of gas passing throughthe furnace. The lower end of the furnace, which is unheated, provides azone wherein the compacts are gradually cooled by the stream of gas to atemperature at which they can be conveniently removed and handled.

It is important for the granular refractory material to be pure withrespect to certain impurities, especially iron oxide and silicondioxide, which may act as fluxes and cause the granules to become fusedtogether or to the compacts themselves at the temperatures employed. Therefractory granules should also be free-fiowing and of sufficientlyuniform particle size to minimize packing. The particle size is notcritical, provided that it is small enough so that the granuleseffectively separate the individual uranium dioxide compacts from eachother and from the wall of the furnace, but not so small as to causeexcessive interference with the passage of gas through the furnace. Forthe purposes of the present invention, a particle size corresponding toabout 8-12 mesh has been found generally satisfactory.

The furnace consists essentially of a tube of a refractory material suchas, for example, fused zirconium oxide. It should of course be free fromany volatile impurities which might contaminate the uranium dioxide.

'It is important to establish a temperature gradient in the mixturebetween the upper end and the zone of maximum temperature so that thecompacts are gradually heated to the sintering temperature. It will beapparent that the movement of the mixture of compacts and granulesthrough the furnace will itself establish such a gradient and no otherprecautions are usually necessary. However, more heat may be supplied tothe furnace at or near the sintering zone than at the upper end of thefurnace, if necessary, to establish optimum firing conditions.

The dimensions of the furnace will depend upon its intended capacity andthe output of the heating element used. These in turn will depend uponthe maximum sintering temperature to be attained and the length of time.the compacts are to be kept at or near this temperature.

It will be clear that these are variable factors, and that the design ofa suitable furnace to meet any particular set of conditions is wellwithin the skill of those trained in the art.

The upper portion of the furnace is conveniently heated by means of anelectrical resistance element, either placed externally around thefurnace or actually embedded in the walls of the furnace itself, anarrangement which is feasible with non-conductive furnace materials. Thelower unheated end of the furnace serves both as a cooling zone for thesintered compacts and as a preheater for the stream of protective gaspassing through the furnace.

The rate at which the mixture of compacts and alumina pass through thefurnace is determined by the rate at which the fired mixture is removedfrom the lower end of the furnace. This may be accomplished in a simplemanner by attaching a length of flexible tubing to the lower end of thefurnace. The end of the tube is then closed with a suitable clamp, oreven with the fingers, and opened periodically to permit removing aportion of the fire mixture. Alternatively, the lower end may be aconstriction of such a size that the mixture is allowed to escape fromthe furnace at a rate corresponding to the desired rate of passage ofthe mixture through the furnace. Using the previously described lengthof flexible tubing, the mixture can be withdrawn from the furnace in thefollowing manner without substantial escape of hydrogen or withoutinterrupting its flow through the furnace. With the lower end closed,the flexible tubing is closed off at a second point a suitable distancefrom the end, thus isolating the portion of the mixture to be removed.The lower end of the tubing is now opened and the isolated portion isemptied into a suitable receptacle. When the lower end of the flexibletubing is again closed and the upper constriction opened, the contentsof the furnace move a corresponding distance downwards. As sinteredmaterial is removed from the lower end of the furnace fresh quantitiesof green mixture are added at the top.

Instead of the simple arrangement of flexible tubing or a constrictionat the bottom of the tubing as described above for controlling thepassage of the mixture through the furnace, more elaborate mechanicalvalves are available which may be used for this purpose if desired.

The process of the present invention is useful for sintering compacts attemperatures up to the sintering temperature of the refractory granules.This upper temperature varies depending upon the chemical nature andphysical form of the granules which are used. Aluminum oxide (Alundum)bubbles, for example, begin to sinter at temperatures near 1650 C. Whilezirconia bubbles and sand begin sintering at somewhat lowertemperatures, up to about 1600 C. The temperatures given above areillustrative only, and are subject to some variation depending upon thesource of the granules and the criteria used to determine the sinteringtemperature.

While the invention has been described as applied to the dewaxing andsintering of uranium dioxide fuel compacts, the method is equallyapplicable to the sintering or heat treatment of other solidcompositions and substances, such as the sintered metal and metal oxideobjects commonly fabricated by powder metallurgy. For example, themethod is applicable to the sintering of oxide mixtures which are evenmore refractory than uranium dioxide, the upper limit being determinedby the sintering temperature of the refractory granules themselves and/or the limitations of the furnace. Instead of an atmosphere of hydrogenor steam, the furnace may be operated with an atmosphere of some othergas, such as, for example, nitrogen, oxygen, argon, carbon dioxide, orair, depending upon the nature of the material being sintered. The greencompacts may be prepared by compressing them in a suitable die, or theymaybe prepared by slip casting or by extrusion. Moreover, other bindersmay be used either in addition to or in place of paraffin. For example,other waxes, stearic acid, cellulose derivatives such ascarboxymethylcellulose, and heat-fugitive polymeric materials such aspolyvinyl alcohol and polyethylene glycol are other materials used forthis purpose, while esters of organic acids are frequently used aslubricants for extruded shapes.

From the foregoing description it will also be clear that the process ofthe present invention can be operated either continuously orsemi-continuously using simple equipment. Even without the aid of anymechanical or automatic equipment little handling of the compacts isrequired since the green mixture is easily prepared and the sinteredcompacts are readily separated from the fired mixture by a screeningoperation, whereby the alumina is recovered for use in sinteringadditional compacts. In addition, a suitable furnace is easilyconstructed from standard materials and apparatus, no speciallymanufactured forms or devices being necessary.

Many other variations of the present invention will be apparent to thoseskilled in the art.

The following examples illustrate the invention.

Example 1 A furnace 1 suitable for use with the method of the presentinvention was constructed from a 36 in. length of fused zirconium oxidecombustion tube 3 having an internal diameter of 1% in. Tube 3 wassupported in a vertical position and the upper one-half of its lengthwas heated by surrounding it with a heavy duty resistance heater 5having a silicon carbide element 7. To control the rate at which thecontents of the furnace gravitate through tube 3, a length of rubbertubing 9 was attached to the lower end of combustion tube 3 and closedwith a series of pinch clamps 11. An inlet 13 for the introduction ofhydrogen was also provided at the lower end of combustion tube 3 aboveclamps 11 closing the rubber tubing.

The furnace 1 was next filled with fused alumina bubbles 15, i.e.,hollow spheres 8-12 mesh in size, and heated to an operating temperatureof 1520 C. Hydrogen was introduced into gas inlet 13 at a ratesufficient to exclude all air from the furnace, particularly from thezone at the upper end of the combustion tube where the pellets areheated to the sintering temperature. Bubbles 15 were withdrawn from thebottom of the furnace by manipulating pinch clamps 11 in the mannerpreviously described and recycled to the top of the furnace to establishthat they would flow satisfactorily.

When the furnace had reached its operating temperature, green uraniumdioxide compacts 17, which had previously been dewaxed, were dropped ontop of alumina bubbles 15 at 15-minute intervals along with additionalbubbles. These particular compacts were cylindrical in form and had alength of 2 cm. and a diameter of 1 cm. At the same time, aluminabubbles 15 were withdrawn from the lower end of the furnace at such arate that compacts 17 were drawn down into the firing zone of thefurnace at the rate of about 16 inches per hour. Under these conditions,the residence time of the compacts in the firing zone was approximatelyone hour. The density of the compacts fired in this manner was 10.0-10.2g./cc.

There was no significant attrition of the compacts during the firingprocess, nor was there any plugging of the combustion tube duringoperation. It was also evident that much less power was required toattain a given firing temperature by this method than if the compactshad been fired 'm an open tube.

Example 2 Green uranium dioxide compacts 17 similar to those describedin Example 1 but containing 0.5% parafiin and 1% polyvinyl alcohol werealso fired in the same furnace. In this case the maximum temperature ofthe firing zone was 1500 C. and the compacts were passed through thefurnace at the rate of about 8 inches per hour. Sintered compacts havinga density of 9.0-9.5 g./cc. were produced in this manner.

If the rate at which the compacts passed through the furnace wasincreased to 16 inches per hour, there was appreciable cracking of thefired compacts, probably because of excessively rapid heat-up with thegeneration of high internal pressures in the compacts.

Example 3 Example 1 was repeated except that the refractory granularmaterial 15, which serves as a support for the uranium dioxide compactsduring the sintering operation, was composed of fused zirconium dioxiderather than fused aluminum oxide. The granules were solid and of a sizecomparable to those employed in Example 1. The results were essentiallycomparable to those obtained when the granular refractory material wasfused aluminum oxide.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above meth ods without departingfrom the scope of the invention, it is intended that all mattercontained in the above description or shown in the accompanying drawingshall be interpreted as illustrative and not in a limiting sense.

We claim:

1. The method which comprises adding freeflowing granules of refractorymaterial having dispersed therein unsintered compacts of at least onepowdered metal substance selected from the group consisting of metallicoxides and metals, which compacts sinter substantially below thesintering temperature of the refractory granules, to the top of avertical, substantially tubular furnace filled with the mixture, onlythe upper portion of the furnace being heated; removing the mixture ofcompacts and granules from the lower end of the furnace at a controlledrate so that the mixture slowly gravitates through the furnace and isthereby gradually heated to a temperature sufiicient to sinter thecompacts but insufficient to sinter the refractory granules and thengradually cooled; and thereafter separating the sintered compacts fromthe refractory granules.

2. The method which comprises adding free-flowing granules of refractorymaterial having dispersed therein unsintered compacts of powdereduranium dioxide, which compacts sinter substantially below the sinteringtemperature of the refractory granules, to the top of a vertical,substantially tubular furnace filled with the mixture, only the upperportion of the furnace being heated; removing the mixture of compactsand granules from the lower end of the furnace at a controlled rate sothat the mixture slowly gravitates through the furnace and is therebygradually heated to a temperature of at least 1500 C. but insufiicientto sinter the refractory granules and then gradually cooled; andthereafter separating the sintered uranium dioxide compacts from therefractory granules.

3. The method which comprises adding free-flowing granules of refractorymaterial dispersed therein unsintered compacts of powdered uraniumdioxide and a heat-fugitive binder, to the top of a vertical,substantially tubular furnace filled with the mixture, only the top ofthe furnace being heated and the furnace further being provided withmeans for passing a flow of gas through the furnace countercurrent tothe direction of movement of the mixture; removing the mixture ofcompacts and granules from the lower end of the furnace at a controlledrate so that the mixture slowly gravitates through the furnace and isgradually heated to a maximum temperature of approximately 1500 'C.,during which time the heat-fugitive binder is driven off and removedwith the outgoing gas, and then gradually cooled; and thereafterseparating the sintered uranium dioxide compacts from the refractorygranules.

4. The method of sintering compacts of a metal substance which comprisessurrounding the compacts with free-flowing granules of a more refractorymaterial, allowing the mixture of compacts and granules to gravitatethrough a vertical, substantially tubular furnace wherein the compactsare gradually heated to a temperature sufiicient to sinter the compacts,but insufficient to sinter the refractory granules, gradually coolingthe mixture and separating the sintered compacts from the refractorygranules.

References Cited in the file of this patent UNITED STATES PATENTS UNITEDSTATES PATENT OFFICE CERTIFICATIGN 0F CORRECTION Patent No; 2,968,551January 17, 1961 Edward Do North et a1,

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 3, line 39, for "fire" read fired column 6, line 2 1 after"material" insert having Signed and sealed this 13th day of June 1961a 3EA L) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of PatentsUNITED STATES PATENT OFFICE CERTIFICATION 0F CORRECTION Patent Nod$968,551 January 17, 1961 Edward D, North at aly It h'ereby certifiedthat error appears in the above numbered patent requiring correction andthat the said Letters Patent should read as corrected below.

Column 3, line 39, for "fire" read fired column 6, line 24!: after"material" insert having Signed and sealed this 13th day of June 1961SEAL) Attest:

DAVID L. LADD ERNEST W. SWIDER Attesting Officer Commissioner of PatentsUNITED STATES PATENT OFFICE CERTIFICATION OF CORECTIO'N Patent Nod 2368551 January 17, 1961 Edward D, North e1; alo

It is h'ereby certified that error appears in the above numbered patenforequiring correction and 'that the said Letters Patent should read ascorrected below.

Column 3, line 39, for "fire" read fired column 6, line 241, after"material" insert, having t.

Signed and sealed this 13th day of June 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents

1. THE METHOD WHICH COMPRISES ADDING FREE-FLOWING GRANULES OF REFRACTORYMATERIAL HAVING DISPERSED THEREIN UNSINTERED COMPACTS OF AT LEAST ONEPOWDERED METAL SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF METALLICOXIDES AND METALS, WHICH COMPACTS SINTER SUBSTANTIALLY BELOW THESINTERING TEMPERATURE OF THE REFRACTORY GRANULES, TO THE TOP OF AVERTICAL, SUBSTANTIALLY TUBULAR FURNANCE FILLED WITH THE MIXTURE, ONLYTHE UPPER PORTION OF THE FURNACE BEING HEATED, REMOVING THE MIXTURE OFCOMPACTS AND GRANULES FROM THE LOWER END OF THE FURNACE AT A CONTRLLEDRATE SO THAT THE MIXTURE SLOWLY GRAVITATES THROUGH THE FURNACE AND ISTHEREBY GRADUALLY HEATED TO A TEMPERATURE SUFFICIENT TO SINTER THECOMPACTS BUT INSUFFICIENT TO SINTER THE REFRACTORY GRANULES AND THENGRADUALLY COOLED, AND THEREAFTER SEPARATING THE SINTERED COMPACTS FROMTHE REFRACTORY GRANULES.